This study will enroll a maximum of 140 patients (with an expected 10% dropout rate) with A1c values between 7.5-10% who will be randomized in a 1:1 fashion to either the study drug or placebo. 100 of these patients will wear a continuous glucose monitor. Subjects may be using insulin via continuous subcutaneous insulin infusion (CSII) or multiple daily injections (MDI). The clinic visits will be conducted at screening, -1 week, baseline/randomization, 4 weeks, 12 weeks, and 16 weeks in addition to telephone visits (Appendix 1 and 2). At home, starting at week 0 (randomization/baseline), subjects will take 100 mg/day of sitagliptin or placebo and continue for the study duration. Laboratory analysis will be performed at various time points assessing glucagon, A1c, C-peptide, glucagon-like-peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP), and insulin levels.